1. Field of the Invention
The present invention relates to a communication apparatus, a method, and a tangible machine-readable medium thereof. More particularly, the present invention relates to a communication apparatus, a method, and a tangible machine readable medium thereof for processing the ranging interference of an orthogonal frequency division multiplexing (OFDM) signal.
2. Descriptions of the Related Art
Orthogonal frequency division multiplexing (OFDM), especially a variant known as orthogonal frequency division multiplexing access (OFDMA), is a wireless transmission scheme that combines time division multiplexing (TDM) and frequency division multiplexing (FDM). This technology employs different subcarriers to provide data transmission for different users or to transmit data for different purposes.
The IEEE 802.16 (also referred to as Worldwide Interoperability for Microwave Access, abbreviated WiMAX) standards specify wireless transmission employing the OFDMA transmission scheme. In the uplink transmission, ranging symbols may be transmitted simultaneously with data from subscriber stations (SSs) to attain and maintain subcarrier synchronization and symbol timing synchronization between the base station (BS) and each subscriber station to avoid interference among the data transmissions of multiple SSs.
When a new-coming SS desires to perform network entry to a BS, the SS will firstly transmit an initial ranging symbol to the BS for initial network entry. However, because the SS has not entered into the network of the BS yet, it does not know the distance from the BS. Consequently, the initial ranging symbol will experience unspecific round-trip transmission delay (RTD). Such a delay will cause the BS to receive an incompletely overlapped ranging symbol in some OFDM/OFDMA symbol periods, and in turn cause interference to data from other SSs due to such incomplete ranging symbols. Such interference that arises from the RTD characteristics of initial ranging symbols of newly coming SSs is called “ranging interference”. FIG. 1 is a schematic diagram illustrating how initial ranging symbols interfere with SS data. Here, the SS data 11 comprises three OFDM symbols 111, 113, 115, with respective cyclic prefixes (CPs) 110, 112, 114; while the initial ranging signal (interchangeably termed ranging code in this invention) 13 comprises two ranging symbols 131, 133. As can be seen in FIG. 1, the OFDM symbols 111 and 115 of the SS data 11 are respectively superimposed with incomplete ranging symbols 135 and 137 of the ranging signal 13, which become sources of interference to the SS data 11.
To prevent ranging interference, a conventional practice is to use a CP with a length larger than the BS coverage area as a guard interval. However, an overlong CP may degrade the bandwidth efficiency. For example, the transmission bandwidth of a WiMAX OFDMA under the 1024-point Fast Fourier Transform (FFT) specifications is 10 MHz. A CP with a length of ⅛ of a symbol may guard up to about 1.75 kilometers (km) of RTD of ranging signals, while a CP with a length of ¼ of a symbol may guard up to 3.5 km of BS coverage area. However, the ⅛-symbol CP is 16.7% better in bandwidth efficiency than the ¼-symbol CP. Further, if a BS coverage area of 10 km in radius is considered, a CP of at least 70% of one OFDM symbol in length will be needed, which will cause a significant degradation of the bandwidth efficiency and is incompatible with the practice in wireless communication systems.
To obviate the degradation in bandwidth efficiency due to long CP, other methods can be employed to mitigate the ranging interference from initial ranging symbols to SS data. One approach is to avoid the ranging interference, while another is to eliminate the ranging interference. Concerning avoidance of ranging interference, a more robust coding and modulation scheme that is less susceptible to interference may be used in the presence of initial ranging symbols, e.g., using 4-QAM modulation or QPSK modulation with rate-1/2 coding to resist the possible ranging interference from initial ranging symbols. Alternatively, all ranging symbols may be concentrated within some transmission slots and separated from the data transmission slots to prevent possible interference from the ranging symbols to the SS data. On the other hand, concerning elimination of ranging interference, a technique that may be termed coherent interference estimation can be used to remove the ranging interference. However, the coherent interference estimation needs to calculate the channel response for each ranging symbol and generate a corresponding ranging interference to remove or correct the ranging interference of each ranging symbol and thereby estimate the data transmitted by the SSs. Although the aforesaid techniques may mitigate the degradation in bandwidth efficiency caused by long CP, they also come with other shortcomings.
In summary, although the solutions to ranging interference described above can all decrease the interference from initial ranging symbols to SS data, they all have their shortcomings. For example, use of a long CP may decrease the overall bandwidth efficiency, using a more robust coding and modulation scheme that is less susceptible to interference may lower the data transmission rate, and use of coherent interference estimation to remove the ranging interference may lead to intensive computations.
Therefore, efforts still have to be made by the wireless communication network operators and the wireless communication apparatus manufacturers to avoid or eliminate ranging interference from the initial ranging symbols without compromising the overall bandwidth efficiency, data transmission rate and data computation complexity.